The present invention concerns in general an optical arrangement for the reflecting microscopic examination of an object, and in particular biological tissues and the interior or rear surfaces of organs such as the endothelium layer of the cornea of the eye.
Numerous materials and especially biological objects must be microscopically examined with incident light because they are inaccessible to back light. When structures of objects underneath the surface of a translucent or transparent material are the subject of examination, incident light reflections from the surface are often superposed on the image of such object structures. The intensity of such surface reflections is, in most cases, significantly stronger than the light reflected or remitted by the object structures to be reproduced, because index of refraction of such object structures usually differs very slightly from that of their environment.
A typical example of a reflecting object structure underneath the surface of an organ is the corneal endothelium of the eye, which is located approximately 0.5 to 0.7 mm below the surface of the cornea. Under the usual vertical incident light illumination, the reflection of light from the corneal surface is superposed for the most part on the image of the endothelium. Similar problems are also encountered during the examination of the rear section of the eye because of additional incident light reflections from the lens of the eye. Therefore, there has been a long felt need to suppress or eliminate such interferring reflections especially in ophthalmological instruments.
It is known that surface reflections affecting observation may be reduced by dividing the cornea of the eye being examined into separate zones for illumination and for observation. This division may be effected either in the close vicinity of the eye under examination or in the vicinity of the eye of the observer.
A prior art arrangement is shown in West German Pat. No. PS 323 161, wherein with the aid of a deflecting prism placed in front of the eye being examined, a slit like diaphragm is reproduced on one half of the pupil of the eye. The beam of light passes through the eye lens and illuminates part of the retina, which is then observed through the half of the pupil not illuminated. An overlap of the two halves of the pupil is prevented by the development of a special system for providing parallel beams for illumination and observation. Because the two beams are parallel to each other in front of the eye being examined, no reflections are able to enter the observation beam path from the surface of the cornea. However, it is not possible to observe the front portions of the eye with such parallel beams.
Similar difficulties are encountered in the arrangement shown in West German Pat. No. P 627 621, wherein the illuminating beam is reflected into the eye by means of a deflecting mirror so that parts of the optical system for observation may also be used for the illuminating beam. Adjacent to the edge of the deflecting mirror located on the optical axis of the system, a separating wall is provided, thus affording an additional separation of the two beam paths.
In West German Pat. No. P 394 227, the illuminating beam is again conducted onto the eye to be examined by means of a specially shaped prism in front of the eye, eccentrically placed with respect to the observation beam path. The observation itself takes place through a narrow slit arranged in the vicinity of the eye to be observed. Here, the direction of observation coincides with the direction of the normal to the surface being examined, so that the illuminating beam regularly reflected from the surface of the cornea cannot enter the path of the observation beam.
Another apparatus for suppressing corneal reflections includes placing a contact lens on the eye to be examined. Such an arrangement, described in West German Offenlegungsschrift OS No. 26 05 786, also serves to observe the inside of the eye. The illuminating light is conducted by means of optical conductors, placed at the rim of the contact lens into the eye. Here, interferring reflected beams from the internal surfaces of the eye are directed primarily in the direction of the optical axis of the contact lens so that observation of an object surface free of reflections is possible outside of said axis. Of course, the illumination of front surfaces of the eye is not possible with this arrangement.
A slit lamp type instrument for eye examinations is shown in West German Pat. PS 814 798, wherein the optical axes of the illumination and observation beam paths are in the plane of a normal to the surface of the eye to be examined. The inclination of the optical axes to the normal to the surface may be varied by rotating the observation and the illuminating system around a common axis. This axis is located in the intersection of the two optical axes and extends through the pupil or the object location to be examined inside the eye. No measures are provided for the elimination of interferring reflections during the examination of front sections of the eye. In investigating the rear area of the eye, a contact lens may be placed on the eye to be examined in order to eliminate the refractive power of the lens of the eye.
The difficulties arising during the observation of the front sections of the eye due to interferring reflections from adjacent surfaces, particularly the surface of the cornea, are described in West German Offenlegungsschrift No. 26 50 650. According to this reference, to eliminate the reflections, either the size of the slit-like luminous field may be restricted, or polarized light may be used. With polarized light, the components of the light ordinarly reflected from the surface of the cornea may be suppressed by means of the arrangement of an analyzer in the path of the observations beam. Both of the methods are inadequate for the observation of surfaces underneath but close to the corneal surface, especially of the endothelium, because the overlap of the interferring reflection and the image of the object is in an inverse proportion to the distance between the object reflecting surface and the corneal surface. Additionally, the depolarizing effect produced within the object layer trans-irradiated by the inhomogeneity of the refractive indices is very slight so that the light components passed by the analyzer are most insufficient for observation. While the visual observation is merely rendered very difficult by the superposition of the object and corneal reflections, it becomes entirely impossible when effected by means of electronic imaging (as in a television camera) because the usual automatic brightness control reacts to the brighter reflection. The image of the object, already weak, is then completely suppressed.